The present invention relates to compositions and methods for treating a substrate to promote adhesion, particularly compositions and methods for treating substrates that contain multiple layers of coating compositions.
It is difficult to formulate coating compositions that can adequately adhere to various substrates like untreated steel, galvanized steel and aluminum because different substrates have widely varying surface properties. Primers can be applied to a substrate in order to increase the ability of a coating composition to adhere to the substrate, but a single primer is usually not effective on different types of substrates.
A conversion coating can be applied to a substrate in order to improve a coating composition""s ability to adhere to the substrate. Conversion coatings which deposit as a microthin coating and react with the substrate come in two types. The first and most common type is an aqueous solution of strong mineral acids which enhances adhesion by chemically reacting with the metallic substrates in a process called xe2x80x9cetchingxe2x80x9d. A drawback of aqueous conversion coatings is that it is difficult to uniformly apply one over a substrate.
The second type of conversion coating is a dispersion in organic solvents. For optimum performance, this type of conversion coating often contains heavy metal pigments such as strontium chromate. The negatives of conversion coatings dispersed in organic solvents are twofold. First, they are not effective on a wide range of substrates. Second, they are hazardous to the environment because they often contain heavy metal pigments.
The present invention provides a coating composition that promotes adhesion between a multiple layer coating composition and a substrate and between the respective layers of the multiple layer coating composition. Further, the coating composition of the present invention is easy to apply, can be used on various substrates, and is not harmful to the environment.
The present invention is a coating composition comprising a phenolic resin, an alkoxy silane, and an acid.
The present invention is also a method for coating a substrate comprising the following steps:
a. applying a controlled thickness of a coating composition comprising a phenolic resin, an alkoxy silane, and an acid;
b. optionally applying a primer coating over the coating applied in step (a); and
c. a topcoat over the coating applied in step (a) or in optional step (b).